In a typical die casting operation, molten metal is injected at high pressure into a fixed-volume cavity defined by reusable water-cooled metal dies. Within the cavity, the metal is molded into a desired configuration and solidified to form a product casting. The metal is injected into the cavity by a shot apparatus comprising a sleeve for receiving a charge of the molten metal and a plunger that advances within the sleeve to force the metal into the cavity. Two types of shot apparatus are known. A hot chamber apparatus comprises a shot sleeve immersed in a bath of a molten metal. In a cold chamber apparatus, the molten charge is transferred, for example by ladle, into the shot apparatus from a remote holding furnace.
Zinc-base alloys are commonly formed by die casting, in large part because of a conveniently low melting point. Heretofore, zinc die castings have exhibited a microstructure characterized by soft phases, such as the eta or alpha phases in zinc-aluminum alloys, that lack stability even at moderately high temperatures. As a result, such alloys have had poor high temperature creep resistance that has restricted their use, mainly to decorative parts.
Rashid et al., 4,990,310 discloses a creep-resistant zinc alloy including 4-11 percent copper, and 2-4 percent aluminum. The alloy includes a microstructure with an intimate combination of fine epsilon and eta phases that is particularly resistant to slip. As a result, the product die casting from the alloy exhibits improved strength and wear resistance primarily due to the epsilon phase, but also a dramatically improved creep resistance, particularly in comparison to similar zinc die castings that are substantially epsilon-free.
Commercial zinc alloys (Zamak and ZA alloys) are used mainly for decorative applications. They are rarely used in functional/structural applications because their strength and/or creep properties do not meet requirements. Instead, stronger materials like steel are used to meet specifications. Steel parts are usually machined, whereas, zinc alloys can be die cast to shape.
Furthermore, many automotive and nonautomotive components are required to withstand high forces at high strain rates. At higher temperatures (up to 150.degree. C.) the strain rate sensitivity becomes more important since low melting metals such as zinc alloys usually soften at this temperature. Thus, any increase in strength to offset this softening is an added value.
Some metals and alloys are strain rate sensitive at room temperatures, but, the magnitude of tensile strength increase is small or negligible. Stainless steel and different aluminum alloys have negligible strain rate sensitivity and the increase in tensile strength was minimal. The increase in strength is also very small with increasing strain rate in other types of aluminum alloys. No increase in tensile strength has been found in other nonferrous alloys such as copper and brass.
Different hot rolled steels, increase less than 10% in tensile strength with increased strain rate for the same strain rate range used in our study (10.sup.-5 to 10.degree. sec.sup.-1). When iron and miled steel are tested at higher temperatures (up to 200.degree. C.), the ultimate tensile strength does not increase with increasing strain rate, and they lose their strain rate sensitivity.
A variety of materials are available from which one may attempt to successfully fashion components from. Many automotive components are subject to very high loads, for example during an automobile crash. Many automotive components are subject to high temperatures such as those components under the hood or components which involve high temperature applications under dramatic loading. Conventional wisdom dictates that such components are constructed from relatively expensive, heavy alloys which often require machining.
The present invention overcomes many of the prior art shortcomings.